1. Field of the Invention
This invention relates to the quantitative detection of heavy metals and metalloids in effluent gases, and more specifically, this invention relates to a method for the quantitative detection of heavy metals such as mercury (Hg) in high temperature gases generated from sources such as coal gasifiers, coal-fired electrical generating plants, ore smelters, and waste incinerators.
2. Background of the Invention
Coal-burning power plants, incinerators, oil-burning boilers and power plants, refuse-derived fuel power plants, and gasification systems are sources of effluent streams with mercury and other heavy metals. These metals are toxic. The combustion of low-rank coals such as Powder River Basin sub-bituminous coal and lignites have been shown to form flue gases where the mercury is primarily in the elemental form. In the gasification of coal, mercury is primarily in the elemental form.
Elemental mercury is difficult to capture from a gas stream. For example, elemental mercury is a semi-noble metal, insoluble in water, and is not efficiently captured by carbon. Much of the mercury contained in power plant flue gas is in the elemental form.
In 2005 the U.S. Environmental Agency (EPA) announced the Clean Air Mercury Rule (CAMR) which places permanent limits on mercury emissions from coal-fired utility boilers and establishes a mercury cap-and-trade program. CAMR will be implemented in two phases, with a first phase annual limit of 38 tons in 2010 followed by a final annual limit of 15 tons to be in effect in 2018. The final limit requires an approximately 70% reduction from 1999 emission levels.
The EPA prefers continuous emission monitoring (CEM) for mercury. CEM monitors for mercury often utilize methods to oxidize all of the mercury present within a slipstream of flue gas in order to facilitate the capture and detection of the mercury. Coal-burning power plants have an electrical power capability of 300 GigaWatts (GW) and constitute a potential market of approximately $100,000,000 for CEM. Other markets include incinerators, natural gas pipelines, gasification systems, chemical process plants, and research and health/safety (air monitoring).
Many technologies are being developed for the control of mercury emissions from flue gases. These methods employ sorbents, catalysts, scrubbing liquors, flue gas or coal additives, combustion modification, barrier discharges, and ultraviolet (UV) radiation for the removal of mercury. These removal methods need support in the form of reliable and inexpensive CEM.
There are commercial devices currently available which offer CEM. These devices often include a gold or sorbent trap with subsequent determination of Hg concentration via UV spectrophotometry. Other methods that involve pretreatment of the stream to be analyzed include real time atomic absorption and X-ray fluorescence. See D. S. Zanzow, S. J. Bajic, D. E. Eckels, and D. P. Baldwin, “Real-Time Atomic Absorption Mercury Continuous Emission Monitor,” Review of Scientific Instruments, 74 (8):3774-3783 (2003); K. J. Hay, B. E. Johnson, P. R. Ginochio, and J. A. Cooper, “Relative Accuracy Testing of An X-Ray Fluorescence-Based Mercury Monitor at Coal-Fired Boilers,” J. Air & Waste Manage. Assoc. 56 (May):657-665 (2006); and S. Kellie, Y. Duan, Y. Cao, P. Chu, A. Mehta, R. Carty, K. Liu, W. P. Pan, and J. T. Riley, “Mercury Emissions From a 100-MW Wall-Fired Boiler as Measured by Semicontinuous mercury Monitor and Ontario Hydro Method,” Fuel Processing Technology, 85 487-499 (2004).
The aforementioned methods are often labor intensive, can be slow, and are often costly. In addition, the methods are often prone to numerous interferences. For example, ozone (O3), which may be found in effluent gas streams, absorbs UV, therefore interfering with those processes using UV light to measure and to transform elemental mercury into ions. This phenomenon is described in Y. Li, S. R. Lee, and C. Y. Wu, “UV-Absorption-Based Measurements of Ozone and Mercury: An Investigation on Their Mutual Interferences,” Aerosol and Air Quality Research, 6 (4), 418-429 (2006). Other quenching agents such as O2, HCl, H2O, CO2, SOx, and NOx also must be removed before UV measurements can be carried out.
Surface acoustic wave (SAW) sensors with a gold-coated substrate are used to develop a continuous emission monitoring system for mercury. This is described in “Semi-Annual Technical Progress Report: Surface Acoustic Wave Mercury Vapor Sensor,” Document Number: DE-AR26-97FT34316-002.12 (Jun. 2, 1998), submitted by Sensor Research and Development Corporation, Orono, Maine to U.S. DOE, Morgantown Energy Technology Center, Morgantown, W. Va. However, the gold on gold-coated substrates in SAW sensors can dissolve moieties other than mercury and become contaminated and thus give inaccurate results. The same problem exists with gold used as a trap supra for mercury. Gold is attacked by aqua regia (HCl+HNO3) and hot sulfuric acid (H2SO4), and reacts with ozone to form gold oxide.
Removal of elemental mercury from effluent gas streams via irradiation with UV light is described in C. R. McLamon, E. J. Granite, and H. W. Pennline, “The PCO Process For Photochemical Removal of Mercury From Flue Gas,” Fuel Processing Technology, 87 85-89 (2005); E. J. Granite and H. W. Pennline, “Photochemical Removal of Mercury From Flue Gas,” Ind. Eng. Chem. Res., 41 5470-5476 (2002); E. J. Granite, H. W. Pennline, and J. S. Hoffman, Effects of Photochemical Formation of Mercuric Oxide, Ind. Eng. Chem. Res., 38 5034-5037 (1999); and in U.S. Pat. No. 6,576,092 awarded to Granite et al., on Jun. 10, 2003. The aforementioned McLamon et al. article and the Granite et al. patent are incorporated herein by reference.
U.S. Pat. No. 7,033,419 awarded to Granite, et al. on Apr. 25, 2006 discloses a process to facilitate mercury extraction from high temperature flue/fuel gas via the use of metal sorbents which capture mercury at high and ambient temperatures.
U.S. Pat. No. 6,690,462 awarded to Seltzer on Feb. 10, 2004 discloses a process, system and apparatus to calibrate a continuous emission mercury monitoring system based on plasma emission spectrometry.
U.S. Pat. No. 6,521,021 awarded to Pennline, et al. on Feb. 18, 2003 discloses a process to facilitate mercury extraction from high temperature flue/fuel gas by adsorption onto a thermally activated sorbent produced in situ at the power plant.
U.S. Pat. No. 5,679,957 awarded to Durham, et al. on Oct. 21, 1997 discloses a process to monitor mercury emissions by UV spectrophotometry.
U.S. Pat. No. 4,713,547 awarded to Grossman on Dec. 15, 1987 discloses a process to monitor mercury emissions by UV spectrophotometry.
None of the aforementioned articles or patents discloses an inexpensive and reliable method for semi-continuous monitoring of pollutants in effluent gas streams.
None of the aforementioned articles or patents discloses a method for semi-continuous monitoring of mercury and other heavy metals and metalloids which is free from interferences from moieties such as aqua regia (HCl+HNO3) and, independently, ozone.
A need exists in the art for a reliable and interference-free semi-continuous detection method for mercury and other heavy metals and metalloids in effluent gas streams.